The invention relates to a power supply device for an electronic calculator or timekeeping device using a battery as a power source, such as a desk-top type calculator and a wristwatch.
The electronic device using a battery as the power source, for example, a wristwatch, uses a lithium battery with the battery voltage (about 3 V) two times as high as the conventional one. The output voltage of the lithium battery is dropped to a proper voltage and the related electronic circuit is driven by the dropped voltage. In this way, the lifetime of the battery is prolonged and the battery replacing work is saved.
To be more specific, in this type electronic timepiece, 3 V of a lithium battery is dropped to 1.5 V, for example, by a voltage drop circuit. The dropped voltage is supplied to a logic circuit including an oscillator, a frequency divider and a time counting circuit for producing hour data. The hour data outputted from the logic circuit is visualized by a display unit using, for example, liquid crystal elements.
Generally, this type device has a lamp circuit to display the time at night and a sounding device to sound an alarm. The load circuit of those devices are directly coupled with the lithium battery as a drive power source of the logic circuit, in order to obtain more effective time display and alarm sounding.
As described above, in this type of timepiece, the logic circuit is driven by the dropped 1.5 V voltage, so that the lifetime of the battery is two times that of the battery when the logic circuit is driven by the 3 V power source not dropped, if the power loss arising from the voltage drop is not considered. In this point, this power supply system is excellent.
In this power supply system, the voltage to be applied as a drive voltage to the logic circuit is formed by dropping the voltage from the lithium battery by a voltage drop circuit. If the voltage of the lithium battery drops, the dropped voltage from the voltage drop circuit also drops, as a matter of course. This causes various troubles, such as stopping of oscillation of the oscillator and an erroneous operation in the frequency divider and the counter. When the lamp circuit is driven by a switch to energize the lamp, a large amount of current flows therethrough to considerably drop the output voltage of the lithium power source, resulting in the above-mentioned troubles. A great voltage drop also occurs when the sounding device such as a buzzer is driven.
The power consumption by the voltage drop circuit is utterly useless. The voltage drop oscillator for dropping the voltage of the battery is indispensable for the power supply system as mentioned above. The voltage drop by this circuit is not negligible.
Accordingly, an object of the invention is to provide a power supply system which can drop voltage with low power consumption and prevent the logic circuit from stopping or erroneously operating when the power source voltage drops.